Consider a regional power system having a strong internal transmission system transmitting power to another strong regional system on relatively weak Interties. Such a regional power system may experience issues with stability during disturbances, such as short circuits, loss of generation, loss of load, loss of one of the Interties, or any combination thereof. Prevalent practice to the solution of these issues is to include more Interties, increase the voltage to higher voltage levels (such as extra high voltage (EHV) levels or ultra high voltage (UHV) levels), or both. Another approach for better power system stability is to employ protection relays with high operation speed.
Travelling wave protection is one approach for super-high speed protection. There are different types of travelling wave protections, for example, travelling wave pilot protection based on directional comparison, travelling wave current differential protection, travelling wave protection based on distance measurements, etc.
Directional pilot protection is one of the most practical and reliable solutions for travelling wave protection. It only needs a small bandwidth channel to transmit binary information between terminals at end points of a transmission line. It can detect the fault direction based on local measurements, which can block the mal-trip on external fault in reverse direction even with wrong communication.
One travelling wave protection mechanism is RALDA for US Bonneville's 500 kV transmission line. Properties of such a travelling wave protection mechanism are, for example, disclosed in U.S. Pat. No. 3,878,460 (A). U.S. Pat. No. 3,878,460 (A) relates to an arrangement for detecting the direction of a fault from a measuring point. In short, in RALDA the polarities of the first wave fronts of local voltage and current are compared. If the polarities are the same, a backward fault is detected. If the polarities are each others reverse, a forward fault is detected. Protection relays at a terminal will transmit the fault direction to other terminals. If both directions are forward directions, it means that an internal fault has occurred. Otherwise, it means that an external fault has occurred.
However, the security of travelling wave protection mechanisms such as RALDA may be influenced by harmonics. Under some conditions, the harmonics may lead to wrong detection of forward faults at both sides of the protected line, and thereby, it may finally lead to mal-trip according to the directional pilot protection principle. So, there is still a need for an improved protection of a transmission line.